Photochromism is the reversible function of a certain compound that it changes its color swiftly upon exposure to light including ultraviolet light such as sunlight or light from a mercury lamp and returns to its original color when it is put in the dark by stopping its exposure to light. A compound having this property is called “photochromic compound” and used as a material for photochromic plastic lenses.
For the photochromic compound used for this purpose, the following properties are required: (I) the degree of coloration at a visible light range before ultraviolet light is applied (initial coloration) should be low, (II) the degree of coloration upon exposure to ultraviolet light (to be referred to as “color optical density” hereinafter) should be high, (III) the speed from the time when the application of ultraviolet light is started to the time when the color optical density reaches saturation should be high (to be also referred to as “high color development sensitivity” hereinafter), (IV) the speed from the stoppage of the application of ultraviolet light to the time when the compound returns to its original state (to be referred to as “fading speed” hereinafter) should be high, (V) the repeat durability of this reversible function should be high, and (VI) the solubility in a monomer composition as a host material of the photochromic compound should be high so that its dispersibility in the host material in use becomes high when it is cured.
To meet the above requirements, a wide variety of photochromic compounds have been synthesized. Since a chromene compound in particular has high durability and high sensitivity to ultraviolet light, a large number of researches have been made on the chromene compound.
To use the above photochromic compound in a photochromic plastic lens, the following manufacturing methods are known: one in which a polymer film containing a photochromic compound dispersed uniformly therein is sandwiched between lenses to manufacture a photochromic plastic lens, one in which a photochromic compound is dispersed in a polymerizable monomer and then the monomer is polymerized by a predetermined technique to obtain a photochromic plastic lens, one in which a photochromic compound is dissolved in, for example, silicone oil to impregnate its surface with the silicone oil at 150 to 200° C. for 10 to 60 minutes and the surface is further covered with a curable material to obtain a photochromic plastic lens, one in which the above polymer film is formed on the surface of a lens and the surface is covered with a curable material to obtain a photochromic plastic lens, and one in which a coating agent comprising a photochromic curable composition containing a photochromic compound and a polymerizable monomer (to be simply referred to as “coating agent containing a photochromic compound” hereinafter) is applied to the surface of a lens substrate and cured to obtain a photochromic plastic lens.
Out of the above methods of manufacturing a photochromic plastic lens, one in which a coating agent containing a photochromic compound is applied to the surface of a lens substrate and cured by photopolymerization is excellent because the most use is made of the characteristic properties of the lens substrate and excellent photochromic properties are obtained.
However, when the inventors of the present invention prepared various coating agents comprising a photochromic compound and cured the obtained coating films by photopolymerization, they found that, though the initial coloration of a coating agent containing a certain photochromic compound is little, the photochromic compound deteriorates by light during photopolymerization, the coating film may yellow, and further its photochromic properties deteriorate after that. Particularly chromene compounds which develop a yellow color (may be referred to as “yellow compounds” hereinafter) are used for color control and other purposes but have the following problems.
For example, they found that a coating film formed by applying a coating agent containing a chromene compound represented by the following formula (A) (refer to WO06/045495) yellows after photopolymerization and deteriorates in durability though its initial coloration is little.

Further, although a coating film formed by applying a coating agent containing a chromene compound represented by the following formula (B) (refer to WO03/082849) rarely experiences initial coloration and does not yellow after photopolymerization, its color optical density is low and its fading speed is too high. Further, its developed color is orange, and it is desired that its absorption wavelength should be made short to use it as a yellow compound.

Although a coating film formed by applying a coating agent containing a chromene compound represented by the following formula (C) has a high color optical density and a moderate fading speed, its initial coloration is marked. When a photochromic plastic lens is manufactured by using this chromene compound, the transmittance of the photochromic plastic lens before exposure to ultraviolet light becomes low.
